The experiments outlined in this proposal will investigate the functional and physiological organization of the mammalian circadian timing system. The circadian rhythms of core body temperature and rest-activity will be continuously monitored in free-ranging hamsters before and after: activity rhythm "splitting" induced by exposure to constant illumination, lesions of the suprachiasmatic nuclei, removal of the adrenal of pituitary glands and controlled infusion of cortisol, estradiol and melatonin. The effects of these manipulations on the circadian control of body temperature and activity will be determined by analyses of the period, phase, strength, waveform and precision of each rhythm before and after experimental treatment. These data will not only allow the first critical evaluation of the relationship between the two dominant models of mammalian circadian organization but will also serve to examine their underlying physiological mechanisms. These studies will provide important new information on how the circadian timing system regulates the temporal organization of mammalian behavior and physiology.